The proposal outlines a series of investigations which aim to provide information essential to the understanding of electron transfer and energy transduction in the cytochrome bc1 region of the mitochondrial respiratory chain. The proposal utilizes a variety of biophysical and biochemical approaches to provide structural and thermodynamic information and to investigate the possibility of functional interactions between components. Structural information will be obtained using oriented multilayer and paramagnetic probes approaches. Oxidation/reduction titrations will be performed under a variety of conditions to evaluate the possibility of cooperativity between various components and to study the causes and implications of spectral shifts. This area is important because mitochondria are the major source of energy for animal cells, because related electron transfer systems are important in a wide variety of systems not directly connected to energy transduction (e.g., steroid hormone synthesis, macrophage peroxide production) and because bacterial and fungal respiration utilize similar respiratory chains.